Comparison of ISM (cold) and stellar wind (warm) X‐ray cross sections for a typical early O star (T_eff = 40,000 K). Note the large number of wind absorption edges and the relative flatness of the wind cross sections between 0.1 and 1 keV. Above 0.6 keV, the ISM and wind cross sections are essentially the same. The C and O K‐shell edges are indicated. Comparing the ISM and wind cross sections we notice an energy shift (~0.08 keV) of the O K‐shell edge, which is a reflection of the higher ionization state of the wind.